As a light source of a projector, a discharge lamp such as a high-pressure mercury lamp or metal halide lamp may be used. In such a discharge lamp, the melting performance decreases in accordance with the wear of electrodes due to discharge, the progress of crystallization of the electrodes accompanied by an elapse of the accumulated lighting time, or the like, whereby the shapes of the electrodes change. In addition, when a plurality of protrusions is grown in the front end portions of the electrodes or electrode main bodies are worn irregularly, a shift of an arc starting point and a change in the arc length occur. These phenomena cause reduction in luminance of the discharge lamp and shorten the life of the discharge lamp, which are not desirable.
As a method of solving the above-described problems, a discharge lamp lighting device that drives a discharge lamp by using AC currents having different frequencies is known.
However, when the discharge lamp is driven by using AC currents having different frequencies as in JP-A-2006-59790, a steady convection current is formed in accordance with light emission inside the discharge lamp. Accordingly, uneven segregation of electrode materials occurs. Therefore, there is a problem in that abnormal discharge may occur at the time of start of lighting and damages in a sealing body (glass container) forming a discharge space may occur.
In addition, by only changing the frequencies, the melting states of the front ends of the electrodes are not sufficient for preventing unevenness, and protrusions formed on the front ends of the electrodes may be deformed. In addition, in a case where the electrodes are excessively melted for maintaining the shapes of the protrusions, the electrode materials excessively vaporize, whereby a problem such as blackening may occur.